Stranica 145 [145]
COLOURS IN STILL WATER 83
other theory explains the colour by supposing that water
has a selectzve absorption for the rays of the red end of the
spectrum—¢hat water is in fact a blue transparent medium.
Three different methods were adopted of testing the cor¬
rectness of these rival theories, and all three proved the
water of the Mediterranean to be blue by selective absorp¬
tion, and show that light in passing through the water has
the rays of the red end of the spectrum absorbed, and only
those of the blue end transmitted.”
In the same paper Dr. Aitken shows how, while the selec¬
tive absorption of the water determines its colour, the
brilliancy of colour is determined by the quantity of solid
particles that the water carries in suspension. All the dif¬
ferent colour phenomena of the Mediterranean are not only
thus easily accounted for, but can even be imitated experi¬
mentally by means of a solution of Prussian blue and a fine
white powder. “If the solution of Prussian blue is placed
in a vessel, the bottom and sides of which are dark and
reflect no light, then the coloured solution appears dark and
colourless; but if a little of the white powder is added then
the solution at once becomes brilliantly coloured. By vary¬
ing the amount of the powder in the water all the varied
colour effects of the Mediterranean can be reproduced, a little
powder causing the solution to appear deep blue, and as more
powder is added the brilliancy of the water increases, and
its colour changes from blue to chalky blue-green.”
The influence of the colour of the floating particles them¬
selves on the appearance of the water is also pointed out,
and the greenness of our northern seas attributed in part to
the reflecting particles being yellow. Dr. Aitken’s investi¬
cations included the examination of well waters, which were
found to vary in colour from blue to yellowish-brown, and
it was observed that the more transparent a water was, the
nearer its colour was to blue. A series of experiments were
also made to determine the colour of distilled water, which
proved to be almost exactly that of Prussian blue. “As the